A carbonaceous biomass raw material can be subjected to conditions converting same into gases. One exemplary gas is syngas which includes carbon monoxide and hydrogen as components. Industrially, gases thus obtained can further be refined to hydrocarbons or other organic compounds. To optimize the yield and reduce or avoid irregularities in the production process, further steps can be employed.
One exemplary step is increasing the hydrogen-to-carbon monoxide ratio in the feed of the synthesis reactor. A strategy for this is a water gas shift (WGS) reaction. See, for example, C. Ratnasamy and J. P. Wagner, Water Gas Shift Catalysis, Catalysis Reviews, 51: 3, 325-440 (2009). In WGS, water and carbon monoxide can react to form hydrogen and carbon dioxide.
When applying a WGS process, one can choose between or use combinations of, for example, four options: high temperature shift (HTS), medium temperature shift (MTS), low temperature shift (LTS) and sour gas shift. Each option can have exemplary conditions depending on the catalysts used. An HTS reactor can have a temperature range of 350-600° C. and the exit gas can have a CO level of a few vol-%. In some applications, the CO level can be further decreased with a LTS reactor after the HTS. Because the temperature range of the LTS can be between 150-300° C., an inter-stage cooler can be used. After the LTS reactor, CO level can be even less than 1 vol-%.
Depending on the source of the syngas, the gas mixture may contain significant amounts of impurities influencing the WGS reaction. Also, side reactions can produce unwanted compounds even from pure syngas. Considering the choice of a catalyst, sulfur compounds can be relevant, because WGS catalysts can have a very poor sulfur tolerance. Two ways to deal with sulfur include, for example, removing same from the feed gas prior to WGS process, or using a sulfur tolerant WGS catalyst. When sulfur tolerant catalyst is applied, the WGS process can be referred to as a sour gas shift reaction.
For sour gas shift reactions, it can be difficult to find suitable catalysts that are both active and tolerate sulfur. A catalyst that can be used is a CoMo-catalyst within temperature range between 230-470° C. These catalysts can have a very good sulfur tolerance. For example, sulfidation may be employed. Their activity in the WGS may not be as good as that of LTS catalysts in the sulfur-free feed gas. Their activity can be dependent on successful presulfiding. According to M. V. Twigg, Catalyst Handbook, 2. ed., Wolfe Publishing Ltd, Frome 1989, 608, p. 306, catalysts applied for HTS, for example, FeCr catalysts, can also be active in sulfided form, but the activity thereof can reduce to 50% of the original HTS activity.
Proper functioning of sulfided catalysts can depend on a minimum sulfur level. With syngas of, for example, biomass origin, sulfur content may vary depending on the raw material batches. For example, the sulfur content may even be too low for the sour gas shift catalyst desirable conditions. In these cases, adding sulfur derivative to WGS reaction to provide a sufficient sulfur level can increase the burden to remove the same within subsequent reaction steps.
The water gas shift reaction can be designed for syngas originated from coal or natural gas. The composition, for example, amounts of the main components, impurities and trace components, can differ depending on the origin of the raw material and can have characteristics originating from the biomass used. For example, methods and strategies that are successful for certain raw materials may not necessarily be readily applicable to biomass gasification and further refinement.